I overclocked my CPU and temperature readings say 100°

1. After overclocking my CPU all the programs I tried (like 5 of them MSI afterburner, Core Temp, CPUID and some other one) showed that all my cores are at 100 degrees Celsius. Doesn't budge anywhere.... The temperatures don't seem to really go that high as I went back to the BIOS and it said 60°. But just in case my CPU starts overheating in the future I want to know it. Any ideas guys?

2. I'm using Intel i5 6400 2.7 GHz. I overclocked to 3.5 GHz with 1.2 V. Do you guys think this is healthy overclocking and it will last a couple of years?
I tried 4 GHz at first with 1.3 V and 1.25 V and I also overclocked the RAM a bit from 2000 MHz to like 2600 MHz but went back to 2000 as I didn't need that much FPS and decided that less volts and Hz will lengthen the lifespan of my parts. Am I thinking correctly and should I change my finalized settings? Currently 3.5 GHz 1.2 V. (Ran OCCT test for 1 hr, no problem there)